CLEANUP: fix missing include <string.h> in proto/listener.h
The file proto/listener.h makes use of strdup() but doesn't include
<string.h> so it's sensible to include file ordering.
diff --git a/include/proto/listener.h b/include/proto/listener.h
index 63bef12..1473bfd 100644
--- a/include/proto/listener.h
+++ b/include/proto/listener.h
@@ -22,6 +22,8 @@
#ifndef _PROTO_LISTENER_H
#define _PROTO_LISTENER_H
+#include <string.h>
+
#include <types/listener.h>
/* This function adds the specified listener's file descriptor to the polling